Silicon dielectric films, such as silicon nitride or silicon oxide films, have multiple uses in semiconductor device and integrated circuit designs. Silicon oxide (SiO.sub.2) is commonly used in semiconductor devices as a passivating material to provide physical and chemical protection to underlying circuit devices and components, as an insulation layer, and as a protective layer during fabrication processes. Silicon nitride (Si.sub.3 N.sub.4) ("nitride") is commonly used during the fabrication of integrated circuits to provide physical and chemical protection to underlying circuit devices and components. The protective qualities of nitride include moisture resistance, hardness, high dielectric strength, and resistance to oxidation.
An important use of nitride is as a protective layer in LOCal Oxidation of Silicon (LOCOS) processes, which are used to create field oxide areas that isolate device islands on a semiconductor substrate. During the LOCOS process, a layer of nitride is first deposited on a silicon substrate, followed by the deposition of a photoresist layer on top of the nitride. Photolithography combined with selective etching is used to precisely replicate the photoresist pattern in the nitride without etching the overlying photoresist layer. The patterned nitride layer is then used as a mask for the thermal oxidation of the substrate.
LOCOS and other fabrication processes require the use of an etching process that etches silicon dielectric material faster than photoresist material. Etching processes may be either wet etches or dry etches. Wet etches, also called chemical etches, are undesirable for selective silicon dielectric-to-photoresist etching for two reasons: (1) wet processes are difficult to integrate into automated fabrication lines; and (2) wet etching of silicon dielectric requires high temperatures that damage the photoresist layer. Dry etch processes, i.e., etching processes that use gases as the primary etching medium, are preferred formost silicon dielectric etching applications because of their ease of use.
A major disadvantage of dry etching processes is their generally poor selectivity and lack of etch uniformity. Photoresist selectivity is a critical consideration, because of the thin photoresist layers used to define submicron geometries, and the use of stacked layers in many devices. In addition, although a slight amount of overetch is desirable to ensure that all material to be removed has in fact been removed, the complicated structures and high aspect rations typical of high-density devices often result in non-uniform etches that have overetched in smaller areas while underetching larger areas.
Typical dry etching processes attempt to increase the dielectric-to-photoresist selectivity of dry etches by optimizing the etching gas formula, adjusting the overall etch rate, or performing multiple etching steps utilizing different etching gases. For example, it is known to use a two-step process with a first plasma etching step using a fluorine gas, and a second ion bombardment step using a mixture of NF.sub.3 and HBr at high pressure. These known processes have typically achieved at most a 1:1 nitride-to-photoresist etching ratio, and depending on the process parameters, may fail to achieve more than a 1:2 ratio.
There is needed, therefore, a dry etching process for use in semiconductor fabrication that is highly selective for silicon dielectric as compared to photoresist material. Also needed is a selective etching process that requires only one etching step.